1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly to the exhaust gas purifying apparatus having a three-way catalyst and a selective reducing catalyst disposed in the exhaust system of the engine.
2. Description of the Related Art
FIG. 21 show a configuration of a conventional well-known internal combustion engine and an exhaust gas purifying apparatus thereof. Three way catalysts 102 and 103, an air-fuel ratio sensor 104, and a binary-type oxygen concentration sensor (referred to as “O2 sensor”) are provided in an exhaust passage 101 of the internal combustion engine. In this apparatus, an air-fuel ratio feedback control is performed so that the air-fuel ratio detected by the air-fuel ratio sensor 104 coincides with a target air-fuel ratio. The three-way catalyst has a characteristic that the removing rates of carbon hydrate, carbon monoxide, and NOx are high in the state where the air-fuel ratio of an air-fuel mixture burning in the engine is controlled to the stoichiometric air-fuel ratio. Accordingly, the stoichiometric operation in which the target air-fuel ratio is set to the stoichiometric air-fuel ratio is normally performed, and the lean operation in which the target air-fuel ratio is set to an air-fuel ratio leaner than the stoichiometric air-fuel ratio is timely performed for improving the fuel efficiency. Further, by modifying the target air-fuel ratio according to the output of the oxygen concentration sensor 105, the air-fuel ratio is controlled according to the characteristic of the three-way catalyst 102.
FIG. 22 shows an example of operation of the apparatus shown in FIG. 21. Specifically, changes in a vehicle running speed VP, a detected equivalent ratio (a parameter which is proportional to the reciprocal of a detected air-fuel ratio, and takes a value of “1.0” when the air-fuel ratio is equal to the stoichiometric air-fuel ratio) KACT, an O2 sensor output VO2, and an amount GNOxEX of NOx emitted to the downstream side of the three-way catalyst 103, are shown.
When performing the lean operation, the NOx removing rate of the three-way catalyst greatly decreases. Accordingly, the NOx emission amount GNOxEX increases as shown in FIG. 22. Further, since oxygen is stored in the three-way catalyst during the lean operation (t101-t102), the three-way catalyst becomes the oxidizing state to reduce the NOx removing rate immediately after the transition from the lean operation to the stoichiometric operation (t102-t103).
Consequently, the NOx emission amount is conventionally suppressed by restricting the operating condition in which the lean operation is performed. Therefore, improvement in the fuel efficiency is insufficient due to suppressing the NOx emission amount.
On the other hand, a method for promptly eliminating the reduction in the NOx removing rate immediately after returning to the stoichiometric operation from the lean operation is known by Japanese Patent Laid-open No. H10-26040 (JP'040). According to the method shown in JP'040, the air-fuel ratio is set, immediately after the end of the lean operation, to a value which is richer than the stoichiometric air-fuel ratio, to promptly remove the oxygen stored in the three-way catalyst.
FIG. 23 shows an example of the operation to which the method shown in JP'040 is applied. The engine operating condition is the same as that of the example shown in FIG. 22. As apparent from FIG. 23, even if using the method shown in JP'040, the increase in the NOx emission amount GNOxEX immediately after the end of the lean operation cannot be suppressed completely (FIG. 23D, t102-t104), which means that the engine operating region where the lean operation is performed cannot be greatly extended. Further, the fuel efficiency is not greatly improved since the fuel efficiency decreases due to enrichment of the air-fuel ratio (FIG. 23B, t102-t104).
Further, the fuel cut operation in which the fuel injection is stopped during deceleration is adopted for improving the fuel efficiency. However, the NOx emission amount increases immediately after the end of the fuel cut operation, since a large amount of oxygen is stored in the three-way catalyst during the fuel cut operation. Therefore, it is necessary to restrict the condition for performing the fuel cut operation, which raises a problem that the fuel efficiency cannot be sufficiently improved by the fuel cut operation.
Further, the NOx removing rate of the three-way catalyst decreases most greatly when the air-fuel ratio is set to a value in the vicinity of “16”. Therefore, the method in which the air-fuel ratio is changed stepwise (very quickly) from the stoichiometric ratio to the air-fuel ratio of about “20” is adopted to suppress the increase in the NOx emission amount upon transition from the stoichiometric operation to the lean operation. However, according to this control method, there is a problem that the engine output torque greatly changes due to the rapid change in the air-fuel ratio, which consequently degrades drivability of the engine. Therefore, execution frequency of the lean operation is reduced in order to cope with this problem.